Precambrian Research 281 (2016) 639–655

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Precambrian Research

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Paleomagnetism of Mesoproterozoic margins of the Anabar Shield: A hypothesized billion-year partnership of Siberia and northern Laurentia ⇑ David A.D. Evans a, , Roman V. Veselovsky b,c, Peter Yu. Petrov d, Andrey V. Shatsillo b, Vladimir E. Pavlov b,e a Department of Geology & Geophysics, Yale University, New Haven, USA b Institute of Physics of the Earth, Russian Academy of Science, Moscow, Russia c Geological Department, Lomonosov Moscow State University, Moscow, Russia d Geological Institute, Russian Academy of Science, Moscow, Russia e Kazan Federal University, Kazan, Russia article info abstract

Article history: Siberia and Laurentia have been suggested as near neighbors in Proterozoic supercontinents Nuna and Received 5 February 2016 Rodinia, but paleomagnetic evidence has been sparse and ambiguous. Here we present four new paleo- Revised 23 May 2016 magnetic poles from undeformed Paleo-Mesoproterozoic (lower Riphean) sedimentary rocks and mafic Accepted 12 June 2016 intrusions of the northwestern Anabar uplift in northern Siberia. Combining these results with other Available online 17 June 2016 Proterozoic data from Siberia and Laurentia, we propose a tight juxtaposition of those two blocks (Euler parameters 77°, 098°, 137° for Anabar to North America) spanning the interval 1.7–0.7 Ga, consti- Keywords: tuting a long-lived connection that outlasted both the Nuna and Rodinia supercontinental assemblages. Siberia Ó 2016 Elsevier B.V. All rights reserved. Anabar Riphean Paleomagnetism Nuna Rodinia

1. Introduction two cratons were separated by several thousand km (Pisarevsky and Natapov, 2003; Pisarevsky et al., 2008) or tight (Pavlov et al., Proterozoic continental reconstructions are crucial for under- 2002; Metelkin et al., 2007; Evans and Mitchell, 2011). The standing long-term Earth history, but their development has loose-fit hypothesis is inspired primarily due to a perceived incon- occurred over decades with some major components yet unre- gruity between 1.1 and 1.0 Ga poles from the two cratons, but as solved, including precise configurations of the supercontinents will be described further, such a conclusion rests on ages of Siber- Nuna and Rodinia (reviewed by Evans, 2013). Paleomagnetic data ian sedimentary strata with rather poor constraints. Evans and are an integral component of such reconstructions, but the Protero- Mitchell (2011) proposed the two cratons to be tightly joined in zoic database has been dominated by results from Laurentia and Nuna but separating through the interval 1.38–1.27 Ga—the era Baltica (Buchan, 2013). The present study addresses reconstruction of numerous mafic intrusive events throughout Laurentia, Siberia, of Siberia, one of the major Proterozoic cratons. Siberia’s paleogeo- and neighboring Baltica—to achieve the more distant relative posi- graphic relationship to Laurentia has been contentious, with juxta- tion apparently required by the 1.1–1.0 Ga poles. Nonetheless, the positions ranging from Laurentia’s western margin (Sears and matching LIP ‘‘barcode” record spanning 1.7–0.7 Ga from Laurentia Price, 1978, 2000, 2003) to its northern margin in a variety of and Siberia (Gladkochub et al., 2010a; Ernst et al., 2016a) may orientations (Hoffman, 1991; Condie and Rosen, 1994; Frost alternatively suggest a tight fit between the two blocks enduring et al., 1998; Rainbird et al., 1998). as late as 0.7 Ga. A relative lull in tectonic activity or sedimentary In the past 15 years, paleomagnetic data have strongly sup- record (e.g. passive margins) in southern Siberia throughout that ported a mid-Proterozoic location of Siberia near Laurentia’s north- interval could also suggest that margin’s location within a conti- ern margin, such that southern Siberia faced northern Laurentia nental interior (Gladkochub et al., 2010b). It is more difficult to (Gallet et al., 2000; Ernst et al., 2000; Pavlov et al., 2002; apply the same test to northern Laurentia, because that margin is Metelkin et al., 2007; Wingate et al., 2009; Didenko et al., 2009). largely covered by Phanerozoic strata (e.g., Kerr, 1982). An unresolved issue is whether such a fit is loose, in which the The purpose of this paper is to report new paleomagnetic data from nearly pristine igneous and sedimentary rocks of the Anabar uplift in northern Siberia, to assess the aggregate paleomagnetic ⇑ Corresponding author. http://dx.doi.org/10.1016/j.precamres.2016.06.017 0301-9268/Ó 2016 Elsevier B.V. All rights reserved. 640 D.A.D. Evans et al. / Precambrian Research 281 (2016) 639–655 record of Siberia and Laurentia in Proterozoic time, and to propose ingly compelling paleomagnetic evidence for a 20–25° relative a new, static configuration between the two blocks that honors rotation between the Anabar and Aldan blocks during Devonian both the geologic and paleomagnetic datasets. Preliminary data formation of the Vilyuy rift system (Pavlov and Petrov, 1997; from some of the sites described herein were reported by Smethurst et al., 1998; Gallet et al., 2000; Pavlov et al., 2008). Aside Veselovskiy et al. (2009), but our present contribution supersedes from this deformation, the Siberian cratonic interior has remained the western and northern Anabar datasets reported in that earlier tectonically stable other than emplacement of Devonian-Triassic paper. kimberlites and the particularly voluminous Permian–Triassic ‘‘traps” (largely mafic, both extrusive and intrusive; Nikishin 2. Geologic setting et al., 2010). The Anabar uplift, which is the study area of this work, is a The Siberian craton, which assembled at about 1900 Ma (Rosen, broad cratonic arch with Paleoproterozoic crystalline basement 2003), is largely covered by Phanerozoic sedimentary rocks. Pre- (shield) encircled by nonconformably overlying, nearly horizontal cambrian rocks, both crystalline and sedimentary, are exposed and regionally unmetamorphosed, Paleo-Mesoproterozoic sedi- around the craton’s margins as well as two shield areas or uplifts: mentary rocks (Fig. 1). The sedimentary succession begins with Anabar in the northwest, and Aldan in the southeast (Fig. 1). Both clastic strata of the Mukun Group, transitioning upsection to car- shield areas (i.e., exposed crystalline basement) are mantled by bonates of the Billyakh Group (Figs. 1 and 2). The lowest clastic lay- sedimentary cover and inferred to represent larger, internally ers contain detrital zircons as young as 1681 ± 28 Ma (n =8; coherent blocks. Whereas the Aldan block exposes much of its Khudoley et al., 2015), providing a maximum constraint for the pre-Phanerozoic basement architecture, the Anabar block is almost onset of sedimentation. Mafic sills intrude the stratigraphy at sev- entirely covered. In the north-central Anabar block, a gentle domal eral levels, and there are numerous mafic dykes as well. The most uplift exposes the Anabar Shield and an annular ring of Mesopro- common ages for dated intrusions are ca. 1500–1470 Ma (reviewed terozoic (Riphean) sedimentary rocks that are invaded by numer- by Gladkochub et al., 2010a; new data presented in Ernst et al., ous mafic intrusions. Geophysical surveys (e.g., Rosen et al., 2016b), the latter figure being shared by mafic magmatism in the 1994) extend the inferred basement architecture of the Anabar Olenëk uplift about 600 km to the east (Wingate et al., 2009); block beyond its uplifted shield regions, under its sedimentary but many Anabar intrusions are suspected to be related to the Per- cover (also described by Pisarevsky et al., 2008). There is increas- mian–Triassic traps (Bogdanov et al., 1998).

Fig. 1. Regional map of Siberian craton (A), highlighting the location of Anabar Shield study areas (B, C). Paleomagnetic sampling sites are color-coded according to characteristic remanent magnetization (ChRM) group. Filled = normal polarity, open = reversed polarity. D.A.D. Evans et al. / Precambrian Research 281 (2016) 639–655 641

The measurements were made using a 2G-Enterprise cryogenic magnetometer (Paris, Yale), and an AGICO JR-6 spinner magne- tometer (Moscow); heating was done in homemade non-magnetic ovens (Paris), a MMTD-80 (Magnetic Measurements Ltd.) thermal demagnetizer (Moscow), and a TD-48 (ASC Scientific) thermal demagnetizer (Yale). Some specimens were pre-treated by low-temperature immersion in liquid nitrogen to remove multi- domain magnetic components (Borradaile et al., 2004), but we found that such procedure had little effect on the quality of data acquired during the subsequent high-temperature demagnetiza- tion. Directional data were fit in almost all cases with least- squares lines, but occasionally least-squares planes or great circles, according to routines developed by Kirschvink (1980) and Enkin (1994). Reconstructions were made using the GPlates freeware package (Williams et al., 2012).

4. Results

About half of the samples yielded well resolved components of the NRM (Table 1). Most of those samples contained only one or two components (Figs. 3–6), with one clearly defined characteris- tic remanence magnetization (ChRM). Within the sedimentary rocks distant to mapped intrusions, which were red-colored, unblocking temperatures extend as high as 680 °C, indicating near-stoichiometric hematite as the remanence carrier (Fig. 3). Within mafic rocks, unblocking temperatures extend as high as 580 °C, indicating low-Ti titanomagnetite as the carrier (Figs. 4–6). Some mafic specimens display two components of magnetization that are nearly antipodal, (e.g., Fomich site 17, Dzhogdzho trap sites 9-07 and 13-00) indicating either intrinsic self-reversal behavior (e.g., Krása et al., 2005; Gapeev and Gribov, 2008), remanence acquisition over a protracted interval of time spanning a geomagnetic field reversal, or a spurious arti- fact associated with stepwise heating (Shcherbakov et al., 2015). Site-mean directions are listed in Table 1. We applied data qual-

Fig. 2. Schematic stratigraphic column (not to scale) for the northwestern flank of ity filters on the number of least-squares lines and/or circles the Anabar Shield. Vertical lines represent unconformities. Circles represent (the latter counting half, total per site >4) and Fisher’s (1953) sedimentary paleomagnetic sampling horizons. These, and approximate mafic sill 95% confidence radius (a95 < 20°). Characteristic remanence direc- levels (with U–Pb ages from Ernst et al., 2016b), are color-coded for ChRM group tions vary according to lithology (Fig. 7). Sedimentary rocks that and polarity interpretation as in Fig. 1. Other ages are discussed in the text. are distant (i.e., more than a few hectometers) to mapped intru- sions, in general, yield either S-down or N-up ChRMs. There is a 3. Methods systematic shift in declination between the lower sedimentary horizons and the upper units, but the directional shift does not In this study, paleomagnetic samples were collected from both occur along the boundary between the Mukun and Billyakh the lower clastic and upper carbonate stratigraphic units of the Groups. Instead, the shift is localized between the Burdur and Riphean succession, and from numerous mafic intrusions, some Labaztakh Formations, within the upper part of the Mukun Group, of them directly dated by U–Pb ages on baddeleyite (Ernst et al., where a disconformity is recognized regionally (Fig. 2). The ‘‘older 2016b). We report data from six raft trips in the northern flank sedimentary” directional group contains only one polarity, (Fomich River) and western flank (Kotuy, Dzhogdzho, Magan, Ilya, whereas the ‘‘younger sedimentary” group contains two polarities. and Kotuykan Rivers) of the Anabar dome, where strata every- Within intrusive units, ChRMs fall into four distinct groups: (1) where dip less than 5°. The sites were mainly block-sampled, a steep, two-polarity group with W-up and E-down directions, (2) except for portable-drilled samples on the upper reaches of the a moderate-inclination component with mainly N-down directions Kotuykan River. Orientation was achieved by magnetic compass but a single site of opposite S-up polarity, (3) a shallow NE direc- and clinometer, occasionally supplemented by solar compass and tion, mainly downward, from the Fomich River, and (4) a shallow indicating local magnetic variations that match expected IGRF val- SW direction, upward, from the western Anabar region plus the ues to within a few degrees. Representative samples from a subset stratigraphically uppermost site of Fomich River. Group 1 likely of the sites were investigated by optical and scanning-electron represents intrusions from the Permian–Triassic Siberian trap large microscopy. igneous province, based on similarity of directions to published Demagnetization was performed within shielded chambers in results (Pavlov et al., 2011). Group 2 is termed the ‘‘enigmatic” the following paleomagnetic laboratories: Institute of Physics of component, and will be discussed at length, below. Groups 3 and the Earth (Moscow), Institut de Physique du Globe (Paris) and Yale 4 are broadly antipodal, but a formal significance test (McFadden University (New Haven). Following measurement of the natural and McElhinny, 1990) reveals that antiparallelism can be rejected remanent magnetization (NRM), all samples were thermally at the 95% confidence level (i.e., the two groups ‘‘fail” the reversal demagnetized up to 580–680 °C with an average of 12–15 steps test by standard measure; although they are within antiparallelism to isolate the components of the natural remanent magnetization. at slightly more lax 99% confidence limits). Some of the departure Table 1 642 Paleomagnetic data from the northern and western margins of the Anabar Shield, Siberia.

Site abbr. River Lithology Weight Lat.(°N) Long.(°E) n/N GDec GInc k a95 Plat(N) Plong(E) U–Pb geochronology or section geochemistry Older sediments 1-04 Fomich Burdur Fm., pink sandstone 0(PLF) 71.2067 107.2928 10/15 359.3 75.1 45.7 7.2 – – 2-04 Fomich Burdur Fm., pink-cherry sandstone 0(PLF) 71.2425 107.1817 15/15 351.8 78.0 43.0 5.9 – – 3-04(H) Fomich Upper Burdur Fm., distant host rocks to 04-3(D) 0(scat) 71.2767 107.1442 0/10 Unstable – – – – – 13sed-08 Upper Il’ya Fm., red sandstone 1 70.5016 106.1270 12/31 165.7 30.9 19.8 10.0 02.3 119.8 Kotuykan 44sed-08 Upper Burdur Fm., chocolate, cherry-colored ss. 1 70.563 105.883 20/30 156.1 30.0 10.1 10.8 01.8 128.8 Kotuykan 74sed-08 Upper Burdur Fm., red sandstone 1 70.5697 105.8727 6/15 173.5 30.1 28.4 12.8 03.1 112.1 Kotuykan 89sed-08 Upper Burdur Fm., red sandstone 1 70.6468 105.9104 10/16 167.1 19.2 9.4 16.6 09.0 118.8 Kotuykan 34-07 Ilya Il’ya Fm., red sandstone 0(scat) 70.3138 105.8413 0/13 Unstable – – – – –

1-00 Magan Burdur Fm., sandstone 0(scat) 69.97 105.47 0/67 Unstable – – – – – 639–655 (2016) 281 Research Precambrian / al. et Evans D.A.D. Mean 4 sites 165.6 27.7 92.9 9.6 04.1 119.9 K = 113 A95 = 8.7

Younger sediments * * 90.9 4-04 Fomich Labaztakh Fm., cherry-colored siltstone 1 71.3194 107.0375 20/20 016.7 28.0 80.0 3.7 3.0 24-04(H) Fomich Kotuykan Fm., gray limestone 100–350 m away 1 71.6403 107.7739 7/31 194.4 35.2 21.3 13.4 01.6 094.2 from dyke at 04-24(D) 25-04(H) Fomich Kotuykan Fm., variegated limestone 150 m away 1 71.6708 108.0250 11/22 193.0 33.5 12.5 13.4 00.4 095.7 from dyke at 04-25(D) 7-07 Dzhogdzho Kotuykan Fm., red dolostone 0(scat) 70.234 104.172 0/11 Scattered – – – – – 2-00 Magan Labaztakh Fm., red sandstone 0(scat) 70.07 104.92 0/20 Unstable – – – – – 3-00 Magan Labaztakh Fm., sandstone 0(scat) 70.05 104.95 0/7 Unstable – – – – – 4-00 Magan Labaztakh Fm., sandstone 0(scat) 70.07 104.92 0/16 Unstable – – – – – 5-00 Magan Labaztakh and Ust-Il’ya Fms., sandstone 0(scat) 70.07 104.92 0/12 Unstable – – – – – 6-00 Magan Ust-Il’ya Fm. 0(scat) 70.07 104.90 0/10 Unstable – – – – – 7-00 Magan Ust-Il’ya Fm. 0(scat) 70.07 104.88 0/14 Unstable – – – – – 8-00 Magan Ust-Il’ya Fm., sandstone 0(scat) 70.07 104.88 0/16 Unstable – – – – – 9-00 Magan Lowest part of Ust-Il’ya Fm., sandstone 0(scat) 70.07 104.93 0/17 Unstable – – – – – Mean 3 sites 194.7 32.2 393.0 6.2 00.3 093.6 K = 553 A95 = 5.2

Steep W-up/E-down ‘‘Permian-Triassic” (Group 1) * * 171.4 O-08(D) Upper Dolerite sill 0.5 70.6880 105.8291 8/12 271.5 68.9 214.1 3.8 47.9 Kotuykan * * O-08(C,H) Upper Exocontact and host rocks to O-08(D) 0.5 70.6880 105.8291 21/23 276.4 72.5 136.6 2.7 51.1 163.5 Kotuykan * * M-08 Upper Mafic sill 1 70.7022 105.6473 13/15 257.8 76.4 64.6 5.2 61.6 169.3 Kotuykan * * 5-07(D) Dzhogdzho 3-4 m wide mafic dyke, trending 045° 0.5 70.2331 104.1851 6/10 010.9 77.3 82.0 7.4 46.2 97.7 * * 5-07(C) Dzhogdzho Exocontact to 5-07(D) 0.5 70.2331 104.1851 4/10 359.5 70.4 47.5 13.5 34.8 104.5 7-07 Dzhogdzho Kotuykan Fm., red dolostone (remagnetized by 1 70.234 104.172 9/10 079.7 70.5 19.6 11.9 53.4 176.7 nearby, unmapped P-Tr intrusion?) 13-00 Dzhogdzho Mafic dyke, trending 030° 1 70.5025 104.4367 9/15 071.2 81.0 26.5 10.2 68.6 156.1 9-07 Dzhogdzho Weathered mafic dyke, 15 m wide, trending 030° 1 70.5028 104.4396 17/21 118.3 79.8 51.7 5.0 56.4 137.1 * * 10-07 Dzhogdzho Mafic sill, 4 m thick 1 70.5028 104.4396 6/12 222.2 71.3 168.6 5.2 66.8 211.4 Mean 7 sites 089.3 78.4 48.9 8.7 61.0 155.6 K = 15.1 A95 = 16.1

N-down/S-up ‘‘enigmatic component” (Group 2) VR1-08 Upper Large (>10 m thick), fresh mafic sill at river level 0.2 70.5155 106.1041 8/8 023.0 67.7 65.0 6.9 67.5 245.8 Kotuykan VR2-08 Upper Same sill as VR1, sampled 3 m higher 0.2 70.515 106.102 8/8 032.8 67.3 98.3 5.6 64.6 232.1 Kotuykan VR3-08 Upper Same sill as VR2, sampled 2 m higher 0.2 70.515 106.101 8/8 044.2 63.8 41.8 8.7 57.1 221.9 Kotuykan VR4-08 Upper Same sill as VR3, sampled 2 m higher 0.2 70.515 106.099 7/8 013.9 64.7 46.6 8.9 65.2 263.0 1493 ± 6 Ma (same sill, Kotuykan 400 m away) VR5-08 Upper Same sill as VR4, at its fine-grained margin 0.2 70.5192 106.0625 8/8 028.7 65.8 91.5 5.8 63.7 239.6 1493 ± 6 Ma (same sill, Kotuykan 1100 m away) 105sed-08 Upper Ilya and Burdur Formations, redbeds 1 70.653 105.932 10/12 028.5 42.8 124.3 4.3 41.3 250.7 Kotuykan E-08 Upper Mafic sill or dyke, trending E-W, crossing river 1 70.697 105.642 14/15 026.7 53.3 35.7 6.7 50.4 249.8 Kotuykan valley

36-07 Ilya Coarse, dark green mafic body, 50 m of exposure 1 70.4240 105.5732 10/15 022.2 60.0 91.2 5.1 58.4 252.6 639–655 (2016) 281 Research Precambrian / al. et Evans D.A.D. 38-07 Ilya Mafic sill(?) at least 18 m thick 1 70.4851 105.4752 9/10 008.6 52.0 41.7 8.1 51.8 273.7 11-00 Magan Mafic sill, mapped as P-Tr 1 70.3111 104.4033 11/11 356.9 57.7 108.4 4.4 58.0* 289.0 * 275.0 2-07 Dzhogdzho Fine, green-black gabbro-dolerite sill >15 m thick 1 70.1953 104.1407 13/16 186.5 54.2 20.4 9.4 54.4 5-07(H) Dzhogdzho Kotuykan Fm., red dolostone host to 5-07(D) 1 70.2331 104.1851 3/28 352.6 60.0 10.6 16.6 60.4 295.6 6-07 Dzhogdzho Kotuykan Fm., red dolostone 1 70.2346 104.1749 9/9 329.2 58.3 35.4 11.4 54.9 327.9 15-00 Dzhogdzho Same large mafic sill as 11-07 and 16-00 1 70.4878 104.5219 5/15 037.7 56.2 46.7 11.3 50.9 233.6 (Group 4) 18-00 Dzhogdzho Weathered dyke, trending 025° 0(scat) 70.5228 104.4242 0/15 Unstable – – – – – 4-01 Dzhogdzho Mafic dyke <1 m wide, trending 025° 1 70.53 104.37 8/8 316.4 70.1 68.0 6.8 64.9 356.5 19-00 Dzhogdzho Mafic dyke, trending 030° 0(scat) 70.5417 104.3683 0/15 Unstable – – – – – Mean 11 sites 009.6 59.4 30.7 8.4 60.1 271.9 K = 15.8 A95 = 11.8

NE-shallow (Group 3) 3-04(D) Fomich 50 m wide dolerite dyke, trending 300° 0.5 71.2767 107.1442 14/15 032.3 05.0 59.7 5.2 13.3 253.9 3-04(C) Fomich Contact rocks to 04-3(D), upper Burdur Fm. 0.5 71.2767 107.1442 9/15 034.2 03.2 56.0 6.9 13.8 251.8 5-04 Fomich Dolerite sill 0.5 71.3422 106.9244 10/15 024.5 22.6 16.9 12.1 28.6 259.4 Geochemistry group I; 1483 ± 17 Ma 6-04 Fomich Dolerite sill (same as 04-6) 0.5 71.3408 106.9278 14/14 024.3 17.9 16.2 10.2 26.0 260.0 7-04 Fomich Dolerite sill 0.25 71.3772 106.8511 13/15 024.1 07.0 26.5 8.2 20.4 261.1 8-04 Fomich Dolerite sill (same as 04-7) 0.25 71.3786 106.8400 14/15 020.9 16.7 16.5 10.1 25.8 263.8 9-04 Fomich Dolerite sill (same as 04-7) 0.25 71.3636 106.8056 15/15 020.6 04.0 23.6 8.0 19.4 264.9 Geochemistry group I 10-04 Fomich Dolerite sill (same as 04-7) 0.25 71.3658 106.8142 9/15 009.1 16.5 14.6 13.9 26.8 276.7 11-04 Fomich Dolerite sill 0.5 71.3747 106.7314 10/15 015.8 09.0 21.0 10.8 22.4 269.7 12-04 Fomich Dolerite sill (same as 04-11) 0.5 71.3717 106.7281 10/10 024.1 01.2 83.1 5.3 16.4 261.5 13-04 Fomich Dolerite sill (same as 04-12) 0(scat) 71.3722 106.7161 9/10 001.7 06.3 7.0 20.9 15.5 285.0 14-04 Fomich Dolerite 1 71.3928 106.5356 7/11 023.3 12.6 27.1 11.8 23.4 261.2 15/16-04 Fomich Dolerite sill 1 71.409 106.380 9/22 039.3 07.2 9.4 17.7 17.8 244.8 Geochemistry group II 24-04(D) Fomich 30 m wide dolerite dyke, trending 005° 0(scat) 71.6403 107.7739 7/15 024.6 08.6 11.5 18.6 12.4 262.6 Geochemistry group II D-08 Upper Gabbro-dolerite sill 1 70.7005 105.6013 11/11 039.7 09.1 13.4 12.9 19.2 243.2 Kotuykan 39-07(D) Upper 20 m wide dolerite dyke, trending 320° 1 70.6059 104.9124 14/14 035.7 34.8 27.9 7.7 34.4 243.0 Kotuykan 39-07(C) Upper Baked contact rocks to 39-07(D) 0(scat) 70.6059 104.9124 4/12 010.8 56.3 16.8 23.1 55.8 269.5 Kotuykan 39-07(H) Upper Host rocks to 39-07(D), Kotuykan Fm. 1 70.6059 104.9124 6/9 020.0 30.6 29.7 12.5 34.5 261.5 Kotuykan 643 (continued on next page) 644 Table 1 (continued)

Site abbr. River Lithology Weight Lat.(°N) Long.(°E) n/N GDec GInc k a95 Plat(N) Plong(E) U–Pb geochronology or section geochemistry Mean 9 sites 028.3 14.1 27.4 10.0 23.9 255.3 K = 48.0 A95 = 7.5

SW-shallow (Group 4) 17-04 Fomich Dolerite sill (Tunbl > 500 °C) 1 71.4317 106.2567 7/11 217.6 35.0 58.4 8.0 33.5 062.6 25-04(D) Fomich 10 m wide dolerite dyke, trending 045° 0(scat) 71.6708 108.0250 (4 + 2c)/10 225.0 19.8 11.9 21.0 02.9 063.8 25-04(C) Fomich Exocontact to 25–04(D); green limestone 0 71.6708 108.0250 9/9 251.6 06.1 195.5 3.7 02.8 036.5 (anom) 181-08(DC) Upper Dolerite dyke (trending NNW) and baked contact 1 70.6333 105.2690 12/32 206.9 11.6 24.3 9.0 23.0 076.0 Kotuykan rocks I-08 Upper 5 m wide dolerite dyke 1 70.5895 104.9653 11/12 215.7 19.7 77.3 5.2 25.6 065.4 Kotuykan 40-07 Upper Dolerite sill, at least 18 m thick 0.5 70.5670 104.5259 14/16 223.9 24.2 80.2 4.5 26.1 055.6 Kotuykan

41-07 Upper Dolerite sill, same as 40–07 0.5 70.5634 104.5381 10/12 223.1 27.6 26.1 9.6 28.3 055.9 639–655 (2016) 281 Research Precambrian / al. et Evans D.A.D. Kotuykan 37-07(D) Ilya 12 m wide dolerite dyke, trending 290° 1 70.4206 105.5630 11/12 232.8 17.1 36.4 7.7 20.1 048.6 37-07(C) Ilya Baked contact rocks to 37–07(D) 0(scat) 70.4206 105.5630 4/7 231.4 21.8 25.0 18.7 22.9 049.2 37-07(H) Ilya Host rocks to 37–07(D), lower Burdur Fm. 0(few) 70.4206 105.5630 (1 + 3c)/9 230.5 25.6 194.3 9.0 25.3 049.5 10-00 Magan Mafic intrusion 1 70.2358 104.6681 7/15 227.8 18.8 17.0 15.1 22.4 052.5 1-07 Dzhogdzho Dolerite sill, at least 10 m thick 1 70.1847 104.1219 13/15 214.1 22.3 51.6 5.8 27.6 065.8 3-07 Dzhogdzho Dolerite sill with differentiated center 1 70.1929 104.1194 16/24 224.0 20.3 44.8 5.6 24.3 055.6 ca.1770 Ma, zircon 17.8 32.3 6.4 25.7 069.0 (xenocr.) 4-0712-00 Dzhogdzho Dzhogdzho Fine-grained Mafic dyke, mafic trending sill 300° 10(scat) 70.2281 70.3869 104.1778 104.3358 17/27 0/12 211.7 Unstable – – – – – 8-07 Dzhogdzho Mafic intrusion 0(few) 70.3094 104.3128 (3 + 2c)/15 212.1 18.2 52.3 11.4 25.7 068.7 3-01 Dzhogdzho Dolerite sill 1 70.47 104.45 13/15 219.0 33.9 44.8 6.3 33.1 059.0 14-00 Dzhogdzho Dolerite sill, maybe same as 11–07 and 16–00? 1 70.4878 104.4889 5/16 222.5 20.2 108.7 7.4 24.4 057.6 11-07(S) Dzhogdzho Dolerite sill, same as 16–00 0.333 70.4964 104.5335 9/10 246.3 30.4 79.7 5.8 23.2 031.6 Transitional geochemistry; 1502 ± 2 Ma 11-07(C) Dzhogdzho Baked contact rocks to 11–07(S) 0.333 70.4964 104.5335 5/11 246.9 31.9 44.0 11.7 23.9 030.7 16-00 Dzhogdzho Dolerite sill, same as 11–07 and perhaps 14–00 0.333 70.5117 104.5292 6/15 251.1 32.1 66.2 7.5 22.7 026.5 17-00 Dzhogdzho Dolerite dyke (mapped as P–Tr), trending 030° 1 70.5153 104.5022 12/15 228.9 28.5 40.0 6.9 27.3 049.6 20-00 Lower Dolerite 0(few) 70.5761 104.2278 (2 + 1c)/11 223.5 35.6 40.7 23.0 33.0 053.6 Kotuykan 12-07 Lower Dolerite sill 1 70.5626 104.0303 7/15 219.9 06.8 112.1 5.7 18.1 061.7 Kotuykan 13-07 Lower 5 m thick dolerite sill intruding Ust-Mastakh 1 70.5634 103.8909 16/25 212.6 18.4 22.9 7.9 07.0 071.5 Kotuykan dolostone 6-01 Lower 10 m thick dolerite sill intruding Ust-Mastakh 1 70.53 103.91 9/20 224.2 19.2 40.5 8.2 23.4 055.5 Kotuykan dolostone 7-01 Lower 5–10 m thick dolerite sill intruding Ust-Mastakh 0(scat) 70.53 103.91 0/15 Unstable – – – – – Kotuykan dolostone 8-01 Lower Dolerite sill at base of high cliff 1 70.52 103.88 9/15 213.6 15.6 18.5 12.3 23.9 067.0 Kotuykan 14-07 Lower Dolerite sill, 40 m above river level 1 70.5194 103.8554 10/11 214.5 15.6 68.0 5.9 23.7 066.1 Geochemistry group I Kotuykan 16-07 Kotuy Large, continuous dolerite sill 0.1 70.3088 103.5395 8/8 212.2 22.124.9 130.1129.1 8.14.9 26.429.4 060.0067.0 18-0717-07 Kotuy Large, continuous dolerite sill 0.1 70.314970.3115 103.5373103.5385 6/84/8 208.3219.0 21.4 27.4 13 28.2 071.7 D.A.D. Evans et al. / Precambrian Research 281 (2016) 639–655 645

from antipolarity is due to a declination offset, which may be tempting to interpret as due to vertical-axis rotation between the west Anabar and Fomich areas, but continuity of exposure and near-horizontality of strata in both regions argue against such an interpretation. In addition, the lone Group 4 direction (SW-up) from Fomich has similar declination to the remainder of Group 4 directions from west Anabar. An alternative explanation for the negative reversal test is a significant age difference between the two polarities of remanence, with Siberian plate motion during the intervening time interval. Two U–Pb dated sills with the Group 4 remanence in west Anabar have ages of 1503 ± 2 and

= number of samples in mean direction/ 1502 ± 2 Ma, whereas the Group 3 sill in Fomich area has a U–Pb N / age of 1483 ± 17 Ma (Ernst et al., 2016b). Following this interpreta- n ; ° tion, we treat the Group 3 and 4 directions as distinct from each = 48.6 A95 = 4.6 25.3 061.4 29.4 078.7 31.0 072.9 27.1 075.5 27.1 062.7 Geochemistry group I 30.4 083.9 27.5 061.6 1503 ± 2 Ma 31.1 073.5 28.2 065.3 31.5 076.4 K other in our analysis.

5. Baked-contact tests

We performed several baked-contact tests (BCTs), to the extent allowable by available outcrops along the rivers. 20.0 30.7 5.8 21.9 38.3 12.5 26.0 73.8 6.5 18.4 40.0 9.7 22.4 59.7 10 22.5 19.3 15.9 23.5 55.8 12.4 26.0 72.2 7.9 23.5 65.4 9.5 26.0 40.2 9.6 ) of the 95% cone of confidence about the mean direction; Plat, Plong = virtual

° (1) Fomich River, site 3-04. A 50 m-wide dyke, and Burdur For- mation sedimentary rocks in the exocontact, both give NE- shallow up Group 3 directions. Burdur host rocks at this site

. are magnetically unstable. However, site 4-04, only six km away, has the ‘‘younger sedimentary” Labaztakh direction.

< 5, (PLF) = present local field, (scat) = a95 > 18 This is not a complete BCT, though it is suggestive of primary n remanences. The baked sediment direction matches pre- cisely the dyke direction, not the Group 3 mean, suggestive 21 sites 218.7

precision parameter, radius( of instantaneous baking of the host rocks rather than record- Ernst et al. (2016b) ing a regional magnetic overprint. (2) Fomich River, site 24-04. A 30 m-wide dolerite dyke has a direction that is clearly within Group 3, but has scatter slightly larger than our cutoff filter (a95 > 18°). Contact rocks Fisher’s (1953) were altered and thus were not sampled, but fresh Kotuykan Formation limestone 100–350 m away yielded a stable , a95 = k ‘‘younger sedimentary” direction. Although not a complete baked-contact test, the data suggest lack of pervasive remag-

1 70.4261 103.4277 5/15 201.8 netization in the region. (3) Fomich River, site 25-04. A 10 m-wide dyke carries the SW- shallow, Group 4 direction (determined by 4 least-squares lines plus 2 planes), but Kotuykan Formation green and red

° variegated limestone strata 40 m and 150 m away bear the S-down younger sedimentary direction. The dyke’s exocon- tact rocks are green limestone yielding an anomalous WSW-shallow direction. Although this direction is closer to the dyke remanence than that of the distant host rocks, the aggregate data are only suggestive of a positive BCT. (4) Upper Kotuykan River, site O-08. A subvertical dyke with exposed (i.e., minimum) width of 25 m intrudes Mukun Group sediments that were sampled at distances of 5 m, 10 m, and 20 m from the contact; all subsites share a Per- mian–Triassic Group 1 direction. No distant host rock was = polarity inverted); Geochemistry groups and U–Pb baddeleyite ages are from

⁄ sampled, so the test is incomplete. (5) Upper Kotuykan River, site 181-08. A 15 m-wide dyke and its exocontact have the same SW-shallow Group 4 direction. Distant host rocks were sampled about 500 m away, but their response to thermal demagnetization was chaotic; so the test is incomplete. (6) Ilya River, site 37-07. A 12 m-wide dyke plus exocontact, plus distant Burdur Fm host rock, all give the SW-up Group 4 direction. The ‘‘distant” host rock samples were collected at 2 m and 10 m away from the contact, so the zone of direc- In site abbreviations, (D) = dyke, (S) = sill, (C) = exocontact, (H) = distant host; for zero weights, (aniso = anisotropic), (few) =

Plat, Plong listed as antipole of the mean direction given in the table. tional concordance only slightly exceeds the canonical half- Mean 27-07 Kotuy 15 m wide alkaline dyke, trending 060 26-07 Kotuy Dolerite sill 1 70.3926 103.4287 8/15 206.6 25-07 Kotuy Large, continuous dolerite sill 0.1 70.3443 103.5363 7/8 205.1 24-07 Kotuy Large, continuous dolerite sill 0.1 70.3378 103.5369 5/8 216.5 23-07 Kotuy Large, continuous dolerite sill 0.1 70.3311 103.5411 (5 + 1c)/8 197.2 22-07 Kotuy Large, continuous dolerite sill 0.1 70.3271 103.5430 4/11 217.4 21-07 Kotuy Large, continuous dolerite sill 0.1 70.3238 103.5424 6/8 206.2 20-07 Kotuy Large, continuous dolerite sill 0.1 70.3214 103.5416 5/8 213.9 19-07 Kotuy Large, continuous dolerite sill 0.1 70.3170 103.5392 7/8 203.6 * dyke-width rule for contact remagnetization. The presence Notes: number analyzed (c = great circles); GDec, GInc = mean ChRM in geographic coordinates (degrees); geomagnetic pole latitude, longitude ( 646 D.A.D. Evans et al. / Precambrian Research 281 (2016) 639–655

Fig. 3. Orthogonal demagnetization diagrams and equal-angle stereonet plots of sedimentary samples from sites distant to mapped intrusions. Upper row: younger sedimentary succession from Fomich River valley, Labastakh Formation (left) and Kotuykan Formation (right). Lower row: older sedimentary succession from the Western Anabar region, Ust-Il’ya Formation (left) and Burdur Formation (right). In all orthogonal demagnetization plots, closed symbols lie within the horizontal projection and open symbols lie within the vertical projection. NRM = natural remanent magnetization. All temperatures are in °C.

Fig. 4. Representative orthogonal demagnetization diagrams and equal-area stereonet plots of (A) sites inferred to be of Permian-Triassic age based on steep west-up or east- down Group 1 remanence direction, and (B) sites carrying the enigmatic north-down or south-up Group 2 direction. Symbols as in Fig. 3.

of a N-down Group 2 direction at site 36-07, only 1.2 km intermediary between the Group 2 and 3 directions, but away, argues against widespread regional overprinting. with a large a95 value so it is excluded from both means. (7) Middle Kotuykan River, site 39-07. A 20 m-wide dyke has a The baked-contact test is considered to be inconclusive. NE-shallow Group 3 direction shared by the distant host (8) Dzhogdzho River, site 5-07. A 3–4 m wide mafic dyke has a rock (Kotuykan Formation gray siltstone, 720 m away from steep-up Group 1 (Permian–Triassic) direction. Its exocon- the contact). The dyke’s exocontact has a direction tact has the same steep-up direction, but the distant host D.A.D. Evans et al. / Precambrian Research 281 (2016) 639–655 647

Fig. 5. Representative orthogonal demagnetization diagrams and equal-area stereonet plots of sites carrying the northeast-shallow Group 3 characteristic remanence direction. Symbols as in Fig. 3.

rock (Kotuykan red dolomite) has a N-down Group 2 direc- any of those non-trap ChRM groups, particularly the N-down tion, as does the red dolomite at the next site, 400 m away. Group 2 direction, could be a mixture of other components, as dis- In addition, site 4-07 (dolerite sill) is also only 600 m dis- cussed next. tant from the baked-contact test, and has a SW-up Group 4 ChRM. The combined results from these sites suggest that both the N-down Group 2 direction and the SW-up Group 4 6. Interpretation of characteristic remanence directions direction pre-date the Permian–Triassic intrusion, which itself carries a primary Group 1 remanence. Based on the information presented above, either the domi- (9) Dhzogdzho River, site 11-07 (sill with U–Pb age of nantly N-down Group 2 direction or the nearly antipodal Group 1501.6 ± 1.9 Ma). Both intrusion and exocontact have same 3 and 4 directions could plausibly be primary. The Group 3 and 4 SW-up Group 4 direction. Mafic intrusions of various ages data are similar to those reported from nearly coeval rocks in the are pervasive in this region of the lower Dzhogdzho River, Olenëk uplift in northeastern Siberia (Wingate et al., 2009) and so it was not possible to find host sedimentary rocks unaf- to some directions obtained from dykes of the eastern Anabar fected by their influence and the test is thus incomplete. uplift (Ernst et al., 2000). Among all groups, multiply sampled intrusions yield site-mean directions that are well clustered rela- To summarize, none of the baked-contact tests conclusively tive to the entire spread of data, implying a positive ‘‘secular vari- demonstrate a primary remanence for either the N-D ‘‘enigmatic” ation test” as one would expect from sampling thermal-remanent Group 2 ChRM direction or the NE/SW shallow ChRM directions magnetizations (TRMs) from quickly cooled intrusions (Halls, of Groups 3 and 4. However, there are strong suggestions of pri- 1986). Groups 2–4 are distributed throughout large areas of our mary remanence in Group 3 (Fomich River sites 3 and 24); and sampling region, although Group 2 is restricted to the west Anabar Groups 2 and 4 are both likely older than Permian–Triassic subregion (upper Kotuykan to Dzhogdzho Rivers). Within Groups 3 (Dzhogdzho River site 5-07)—assuming that each group contains and 4, a rough correlation with stratigraphy may be evident, with a pure, uncontaminated representation of the local geomagnetic Group 3 among the lower levels of sill intrusion, and Group 4 field at some time in the past. There remains the possibility that among the higher levels (Figs. 1 and 2). 648 D.A.D. Evans et al. / Precambrian Research 281 (2016) 639–655

Fig. 6. Representative orthogonal demagnetization diagrams and equal-area stereonet plots of sites carrying the southwest-shallow Group 4 characteristic remanence direction. Symbols as in Fig. 3.

The first class of explanations for the difference between the Dzhogdzho sites 15-00 (Gr. 2), 11-07(Gr. 4) and 16-00 (Gr. 4). A N-down ‘‘enigmatic” Group 2 component and the NE/SW shallow similar discrepancy exists at Dzhogdzho sites 1-07 (Gr. 4) and 2- Groups 3 and 4 components assumes that they all accurately 07 (Gr. 2), both from the same intrusion, although the 2-07 ChRM record the local geomagnetic field at the time of remanence acqui- is the only southerly-upward polarization of the Group 2 set—one sition, recording excursions of either Siberian APW or the could choose to consider it as a marginal member of Group 4, but mid-Proterozoic geodynamo. We suspect an APW explanation is then the discrepancy between it and 1-07 from the same intrusion unlikely, because Group 2 is identified in the Upper Kotuykan River remains a challenge for APW or geomagnetic explanations of all the sill site VR1-5 (1493 ± 9 Ma) whereas Group 4 is found in sills of the directional discordances in our dataset. same age, within error: Dzhogdzho River site 11-07 (1502 ± 2 Ma) A second class of explanations for the Group 2–4 directional and Kotuy River sites 16-07 to 25-07 (1503 ± 2 Ma). Stretching discordance invokes rock-magnetic artifacts of either anisotropy the age uncertainties to their limits, one would require 49° of or component mixing. Anisotropic effects are unlikely because APW in a span of only 21 million years, corresponding to a mini- none of the sampled rocks are visibly anisotropic as would be nec- mum 26 cm/yr rate of continental motion. A slight age difference essary to account for the 30° directional discordance. The Group 2 between directional groups could perhaps in principle be detect- direction, being N-down directed at Northern Hemisphere sites, is able by subtle geochemical variations, and indeed there are two inherently suspect as being contaminated by a viscous remanent distinct trace-element geochemical groups of intrusions described magnetization (VRM) acquired in the present Earth field, perhaps by Ernst et al. (2016b); however, those two groups do not corre- partially overprinting Group 3 northeasterly ChRMs. This seems spond to the paleomagnetic directional groups (Table 1). If the pale- unlikely, however, because (a) Group 2 samples exhibited omagnetic discrepancy is to be explained by a geomagnetic straight-line demagnetization trajectories that would require excursion, then the enigmatic Group 2 more likely records the coincidentally identical unblocking spectra between the two anomalous field because it is less abundant across the field area. components, (b) Group 2 sites were typically Fisher-distributed, Nonetheless, it then becomes puzzling why the excursion would with no preferred elongation direction at either the within-site be observed in so many rocks spread across 3000 km2, including or between-site hierarchical level, (c) Site 15-00 has a Group 2 not only mafic intrusions but also redbeds, the latter presumably direction but the adjacent sites 14-00 and 16-00 (the former bearing a thermochemical remanence unlikely to be acquired at possibly and the latter definitely from the same intrusion) have precisely the same time. Most troubling for this class of explana- SW-seeking ChRM directions (Group 4) rather than NE-seeking tion, however, is the fact that both Group 2 and Group 4 directions Group 3 directions, and (d) Group 2 directions in red dolomite are found at different sites within the same intrusion, for example (sites 5-07 and 6-07) and red sandstones (site 105sed-08) render D.A.D. Evans et al. / Precambrian Research 281 (2016) 639–655 649

Fig. 7. Equal-area stereographic projection of all site means reported in this study, color coded by remanence grouping as in Table 1 (color-coded as in Fig. 1). Solid symbols are in the lower hemisphere; open symbols are in the upper hemisphere. Each irregular envelope surrounds multiple sites collected from the same intrusion. U–Pb baddeleyite ages are in Ma (Ernst et al., 2016b). Site 2-07 is discussed in text. S = sedimentary site within Group 2. Star is the present dipole field direction for the sampling area.

a VRM interpretation unlikely because those redbed sites are unli- boundaries to amphibole, chlorite, and epidote. On the whole, kely to contain multi-domain magnetite, the typical carrier of samples from Groups 3 and 4 tend to be less altered than samples VRMs. from Group 2. The enigmatic Group 2 direction could alternatively be In backscattered scanning electron microprobe (SEM) imagery, interpreted as a contaminated partial overprint from Permian– Groups 3 and 4 show variability of Fe-oxide phases, both in grain Triassic traps. It is noted that 8 out of 11 of the Group 2 sites are size and in morphology. Group 2 samples tend to have larger located within 7 km of a site bearing a Permian–Triassic Group 1 amalgamations of Fe-oxide-bearing grains with oxy-exsolution direction. However, the polarity of nearest Group 1 site does not features. Group 1 Fe-oxide grains are smaller and lack distinc- always match that of the proximal Group 2 site. Also, it seems tive internal structure. In all samples, the presence of high- improbable that all five sites in the single sill at the base of the suc- temperature oxidation and solid solution decay is supported by cession along the upper Kotuykan River (VR1-5, dated by U–Pb on SEM observations and microprobe analyses, and these can be baddeleyite at 1493 ± 9 Ma) would be partially remagnetized by a considered as evidence for a primary magmatic origin of the most trap intrusion where none is recognized in that area. of magnetic minerals. Groups 3 and 4 show sufficient variability of Although most samples yielded single-component behavior, magnetic mineralogy to corroborate the idea that recording of the some examples were observed of Group 2 partial overprinting on ambient geomagnetic field occurred over enough time to average either a sedimentary ChRM (Fig. 3, sample 99), or one the Group paleosecular variation. 3 or Group 4 characteristic remanences (Fig. 5, sample 185; Hysteresis parameters of studied samples, summarized on the Fig. 6, sample 7247). There were only rare instances of a Group 3 plotting convention of Day et al. (1977), show that they contain or Group 4 component unblocking at lower temperatures than a single-domain and pseudo-single-domain (SD/PSD) magnetic Group 2 component (Fig. 6, sample 7247). particles, and can be considered as stable magnetic carriers over Petrographic and rock-magnetic results (Fig. 8) may shed some geological timescales (Fig. 8A). The Group 3 and 4 samples exhibit additional light on the origin of Group 2 remanence, although the the greatest variability in hysteresis parameters, whereas Groups 1 data are far from definitive. Group 2 samples tend to be finer- and 2 are confined to the central part of the PSD field. This may grained than those of Groups 3 and 4, with pyroxene and glassy indicate support for briefly emplaced and relatively homogeneous matter completely overprinted by epidote and chlorite, and cre- magnetic mineralogy for each of those two directional groups ation of fine-grained opaque minerals on the chloritized pyroxene. (Permian–Triassic and the enigmatic group). Thermomagnetic Groups 3 and 4 tend to be coarser-grained, with ophitic and curves of bulk susceptibility versus temperature (Fig. 8B) all indi- poikilo-ophitic textures. Partial to complete saussuritization cate dominant presence of near-stoichiometric magnetite, with affects plagioclase, and pyroxene is variably altered along grain Hopkinson peaks at temperatures immediately below 580 °C. 650 D.A.D. Evans et al. / Precambrian Research 281 (2016) 639–655

Fig. 8. Rock-magnetic data from representative samples of the four directional groups, color-coded as in Fig. 1. (A) Plot of hysteresis parameters (Mrs/Ms = ratio of saturation remanence to saturation magnetization; Bcr/Bc = ratio of coercivity of remanence to coercivity) from samples plotted against the canonical fields (Day et al., 1977) of single- domain (SD), pseudo-single-domain (PSD) and multi-domain (MD) magnetite. (B) Selected samples from each of the four characteristic remanence (ChRM) directional groups, showing SEM backscattered imagery (scale bar = 20 lm wide for all images; the bar for VR1-08 is almost too narrow to see, as the field of view is 3 mm wide), and bulk susceptibility versus temperature for heating and cooling.

These curves show more variable behavior for Groups 3 and 4, with ca. 1500 Ma intrusions around the Anabar Shield. This interpreta- some curves largely reversible and others irreversible. Group 2 sam- tion is mainly due to (a) greater abundance relative to the ples are dominated by nearly reversible thermomagnetic behavior, enigmatic Group 2 component, (b) various possible explanations whereas Group 1 shows consistently irreversible behavior. for the Group 2 component including modest degrees of remagne- Altogether, we favor the Group 3 and Group 4 magnetizations tization or acquisition during geomagnetic excursions, and (c) an as most likely to represent primary magnetizations among the overall smooth progression of paleomagnetic poles through the D.A.D. Evans et al. / Precambrian Research 281 (2016) 639–655 651

Fig. 9. New paleomagnetic poles generated in this study, compared to selected published results. Pole abbreviations follow Table 2, plus P–Tr (Permian–Triassic). Aldan block and its generalized ca. 1070–1000 Ma pole path (pink) have been restored to the Anabar reference frame in pre-Devonian time (Pavlov et al., 2008; Euler parameters from Evans, 2009). The individual site-mean virtual geomagnetic poles from Ernst et al. (2000) have italicized numeric labels and U–Pb ages. Dark squares are selected Paleozoic running-mean Siberian (Anabar frame) poles from Cocks and Torsvik (2007). stratigraphic succession, from the older Riphean sedimentary position relative to Laurentia during mid-Proterozoic time. Our units, to the younger sedimentary units, and finally to the Group new Group 3 and Group 4 poles, dated at 1483 ± 17 and 1503 ± 3 and 4 results (Fig. 9). 2 Ma, respectively, can be compared to mid-Proterozoic poles from In addition to these arguments in favor of the Group 3 and 4 Laurentia (Table 2), in particular the St. Francois Mountains data representing the Siberian craton at ca. 1500 Ma, we note the igneous province result from Meert and Stuckey (2002). Our new consistency of the Group 3 and 4 poles with that of the nearly coe- poles conform to earlier suggestions of a juxtaposition between val Sololi-Kyutingde pole from the Olenëk area to the east the southern Siberian and northern Laurentian margins (Fig. 10). (Wingate et al., 2009; Fig. 9). Our new data are similar to some Our new poles are substantially discordant to the St. Francois results from eastern Anabar dykes (Ernst et al., 2000; Fig. 9), but Mountains pole in the Siberia-Laurentia reconstruction models of our poles differ from the Kuonamka mean pole of the same study, Sears and Price (1978, 2000, 2003). which was assigned an age of 1503 ± 5 Ma based on U–Pb dating of The question of whether the Siberia-Laurentia fit was loose one of the dykes. Fig. 9 shows how the five-dyke Kuonamka mean (Pisarevsky and Natapov, 2003; Pisarevsky et al., 2008) or tight lies between the dated dyke’s VGP and both the Olenëk pole and (Pavlov et al., 2002; Metelkin et al., 2007; Evans and Mitchell, our new results. We follow Evans and Mitchell (2011) in interpret- 2011) during mid-Proterozoic time is not addressed directly by ing the dated Kuonamka dyke’s anomalous remanence—relative to our new poles, because they fall atop the coeval Laurentian poles the vastly more abundant Olenëk dataset of Wingate et al. (2009) about equally well in either of the two reconstructions (compare and now also our northern and western Anabar results—as Fig. 10a and b). Other Proterozoic poles from Siberia may aid in representing either a geomagnetic excursion or, perhaps, hitherto answering this question, however. Fig. 10 shows Siberian and Lau- unrecognized (and if so, dramatic) rotations of Siberia at ca. rentian poles from the interval 1750–700 Ma (Table 2). Either 1500 Ma. reconstruction accommodates the poles in a single APW path with reasonable consistency, but the most important difference occurs 7. Tectonic reconstruction in the 1100–1000 Ma datasets. In a loose-fitting reconstruction (Fig. 10a), the Linok and Uchur-Maya poles (Malgina to Kandyk) As noted above, the primary goal of this study is to determine fall along the 1090–1000 Ma portion of the Laurentian APW path; whether new paleomagnetic data from Siberia can elucidate its whereas in the tight-fitting reconstruction (Fig. 10b), the same 652 D.A.D. Evans et al. / Precambrian Research 281 (2016) 639–655

Table 2 Paleomagnetic poles shown in Figs. 9 and 10.

Craton/rock unit Code Age (Ma) Pole(°N,°E) A95(°) 1234567 Q References Siberia (Anabar Ref. frame) Nersa complex A Nersa 1641 ± 8 23, 130 12 1111110 6 Metelkin et al. (2005) and Ernst et al. (2016a) Ilya-Burdur Ily-Bur 1690–1500 04, 120 9 0110100 3 This study, Khudoley et al. (2015) Labaztakh-Kotuykan Lab-Kot

Laurentia Cleaver dykes Cleav 1740 + 5/-4 19, 277 6 1111101 6 Irving et al. (2004) Melville Bugt dykes Melv 1638–1619 03, 261yy 9 1110111 6 Halls et al. (2011) Western Channel diabase WCh ca. 1592 09, 245 7 1101101 5 Irving et al. (1972) and Hamilton and Buchan (2010) St Francois Mtns StFr 1476 ± 16 13, 219 6 1111101 6 Meert and Stuckey (2002) Michikamau intr. comb. Mich 1460 ± 5 02, 218 5 1111011 6 Emslie et al. (1976) Spokane Fm Spok 1470–1445 25, 216 5 1111101 6 Elston et al. (2002) Snowslip Fm Snow 1463–1436 25, 210 4 1111111 7 Elston et al. (2002) Purcell lava Purc 1443 ± 7 24, 216 5 1111101 6 Elston et al. (2002) Abitibi dikes Abit 1141 ± 2 49, 216 14 1111111 7 Ernst and Buchan (1993), excl. A1 Halls et al. (2008) Logan sills Logan 1111 ± 3 47, 218 4 1111111 7 Lulea Working Group (2009)yyy Osler R – lower 3rd OsR1 1111–1108 41, 219 4 1110111 6 Swanson-Hysell et al. (2014a) Mamainse Point R1a MPR1a 1111–1105 50, 227 5 1111111 7 Swanson-Hysell et al. (2014b) Osler R – middle 3rd OsR2 1110–1103 43, 211 8 1111111 7 Swanson-Hysell et al. (2014a) Osler R – upper 3rd OsR3 1105 ± 2 43, 202 4 1111111 7 Swanson-Hysell et al. (2014a) Mamainse Point R1b MPR1b 1110–1100 38, 206 4 1111111 7 Swanson-Hysell et al. (2014b) Mamainse Point N1 + R2 MPmid 1100.4 ± 0.3 36, 190 5 1111111 7 Swanson-Hysell et al. (2014b) North Shore Volcanics N NSVN 1102–1095 36, 182 3 1110111 6 Tauxe and Kodama (2009) Chengwatana Volcanics Cheng 1095 ± 2 31, 186 8 1110111 6 Kean et al. (1997) and Zartman et al. (1997) Portage Lake Volcanics PLV 1095 ± 3 27, 178 5 1111101 6 Hnat et al. (2006) Mamainse Point N2 MPN2 1100–1094 31, 183 3 1111111 7 Swanson-Hysell et al. (2014b) Cardenas basalts + intrus. Card 1091 ± 5 32, 185 8 1110101 5 Weil et al. (2003) Lake Shore Traps LST 1087 ± 2 23, 186 4 1111101 6 Kulakov et al. (2013) Nonesuch Fm None ca. 1065? 08, 178 6 0110100 3 Symons et al. (2013), age interpolated from APWP Freda Fm Freda ca. 1055? 02, 179 4 0110100 3 Henry et al. (1977), age interpolated Jacobsville Fm (A + B) JacAB ca. 1040? 09, 183 4 0110110 4 Roy and Robertson (1978), age interpolated Chequamegon Fm Cheq ca. 1035? 12, 178 5 0110100 3 McCabe and Van der Voo (1983), age interpolated Haliburton A Hal-A 1015 ± 15 33, 142 6 1110000 3 Warnock et al. (2000) Adirondack fayalite granite Ad-fay ca. 990 28, 133 7 1110010 4 Brown and McEnroe (2012) Adirondack metam. anorth. Ad-met ca. 970 25, 149 12 1110010 4 Brown and McEnroe (2012) Adirondack microcl. gneiss Ad-mic ca. 960 18, 151 10 1110010 4 Brown and McEnroe (2012) Tsezotene sills Tzes 780 ± 2 02, 138 5 1110111 6 Park et al. (1989) Wyoming Gunbarrel dikes WyGB 780 ± 3 14, 129 8 1110101 5 Lulea Working Group (2009)yyy Uinta Mtn sandstone Uinta ca. 750 01, 161 5 1110110 5 Weil et al. (2006) Franklin LIP (authochth.) Frank ca. 720 07, 162 3 1111110 6 Denyszyn et al. (2009)

Notes: The seven quality criteria and ‘‘Q” factor are described by Van der Voo (1990). * Unit weight given to each section (N = 2). y Euler rotation parameters of pre-Devonian Aldan block to Anabar-Angara: 60, 115, 25 (Evans, 2009). yy Euler rotation parameters of Greenland to North America: 67.5, 241.5, 13.8 (Roest and Srivastava, 1989). yyy See Pisarevsky et al. (2014).

Siberian poles correspond to slightly older Laurentian APW ages on sedimentation, with the youngest population in basal strata of beginning closer to 1100 Ma. The youngest of these Siberian poles, the Kerpyl Group dated at 1120 ± 17 Ma (Khudoley et al., 2015). Kandyk sills (Pavlov et al., 2002) is well dated at ca. 990 Ma, and With such age constraints, both the loose-fitting and tight-fitting accords moderately well with Grenvillian intrusive poles of about reconstruction options remain viable. the same age from Laurentia (Warnock et al., 2000; Brown We are left, then, with the somewhat unsatisfactory conclusion and McEnroe, 2012)—especially when considering that the that according to Mesoproterozoic paleomagnetic poles, both the paleohorizontal datums of Grenvillian intrusions are not well loose-fitting and tight-fitting reconstructions of Siberia and Lau- established—and also recognizing the caveat that internal Grenville rentia are possible. Each has its prediction of the ages of Kerpyl terranes may be substantially allochthonous (Halls, 2015). The Group strata, via comparison to well dated Laurentian poles. How- older Siberian poles from that interval, Linok and Malgina, are ever, there is one more pole comparison that may shed additional not precisely dated. The Malgina Formation (within the Kerpyl light on this dichotomy of ideas. Recent paleomagnetic study of Group) has a Pb/Pb isochron age of 1043 ± 14 Ma (Ovchinnikova 758 ± 4 Ma Kitoi dykes, in SW Siberia (Pisarevsky et al., 2013), pro- et al., 2001), but it is recognized that such a value represents early duced an excellent match with Laurentian poles in a tight-fitting diagenesis rather than deposition (Kaurova et al., 2010). Recent reconstruction, and a rather poor match in the loose-fitting recon- U–Pb detrital zircon results provide firm maximum constraints struction (Fig. 10). According to the authors of that study, Siberia D.A.D. Evans et al. / Precambrian Research 281 (2016) 639–655 653

Fig. 10. Alternative reconstructions of Siberia in the present North American reference frame. In both models, Aldan is first restored to Anabar for pre-Devonian time (Euler parameters 60°, 115°,25°) according to Evans (2009). (A) Loose fit adopted from Pisarevsky et al. (2014) (Anabar to North America 70°, 133°, 127°). (B) Long-lived, tight fit proposed in this study (Anabar to North America 77°, 098°, 137°). Pole abbreviations are identified in Table 2. Baltica is restored to Laurentia (47.5°, 001.5°,49°) according to Evans and Pisarevsky (2008). migrated from a loose fit to a tight fit during ca. 780–760 Ma considered as a robust estimate of Siberia’s paleogeography at ca. dextral transform motion associated with Rodinia breakup. The 1730–1720 Ma. In southwestern Siberia, mafic rocks of the Nersa model helps explain synchroneity of ca. 725-Ma mafic magmatism complex were once considered entirely Neoproterozoic–Cambrian in both southern Siberia and northern Laurentia (Ernst et al., in age (Gladkochub et al., 2006), but one mafic sill that yielded a 2016a), but it calls for mid-Neoproterozoic strike-slip relative paleomagnetic pole (Metelkin et al., 2005) is now dated by U–Pb motion that is not particularly evident along either margin. We on baddeleyite at ca. 1640 Ma (Ernst et al., 2016a). That pole, with propose a simpler model in which the coincidence of Kitoi poles its new age constraint, actually fits well atop the common Siberia- with those of mid-Neoproterozoic Laurentia merely represents Laurentia APW path in either the loose or tight fit (Fig. 10). the near-final stages of the long-lived pairing between the two cratons in their tight juxtaposition, and that the cratons began 8. Conclusions separating at the time of the 725-Ma magmatism. Our preferred reconstruction (Fig. 10b) is chosen to optimize both the cratonic We produce new paleomagnetic poles from the northern and marginal outlines and paleomagnetic poles from the entire 1.7– western margins of the Anabar Shield, Siberia. Our data, when 0.7 Ga interval, and to honor many geological comparisons compared to similarly aged poles from Laurentia, allow a tight- between the two cratons throughout that history (Evans and fitting juxtaposition between the two cratons, slightly modified Mitchell, 2011; Ernst et al., 2016a). from that of Rainbird et al. (1998) and Evans and Mitchell (2011) If our model of billion-year tectonic stability (within the resolu- and internally stable for about a billion years (1.7–0.7 Ga). The tion of paleomagnetic data) between Siberia and northern Lauren- alternative, loose-fitting reconstruction between southern Siberia tia is correct, then all high-quality paleomagnetic poles must and northern Laurentia (e.g., Pisarevsky and Natapov, 2003; conform to a common APW path between the two blocks for that Pisarevsky et al., 2008) requires transform motion between the interval of time. As shown in Fig. 10, our model accommodates cratons during Rodinia breakup (Pisarevsky et al., 2013); whereas the Mesoproterozoic-Neoproterozoic poles from both cratons. Sev- our model accommodates all of the high-quality poles from both eral recent paleomagnetic and geochronologic results from late cratons without any internal motions. Both models make specific Paleoproterozoic rocks, however, warrant additional discussion. predictions regarding the ages of sedimentation of Riphean strata In the Ulkan graben, rocks from both volcanosedimentary units that have yielded high-quality paleomagnetic poles, and thus fur- and intrusive granitoids yielded paleomagnetic poles (Didenko ther geochronology of those successions may assist in distinguish- et al., 2015). The volcanosedimentary pole from the Elgetey Forma- ing which reconstruction is viable. tion, dated at 1732 ± 4 Ma, passes both fold and intraformational conglomerate tests, but it is highly discordant to other poles from Acknowledgements the Siberian craton and is thus interpreted by the authors as having been deflected by local rotations during graben development. The We thank Andrei Khudoley and an anonymous reviewer, and granitoid pole, with an estimated age of 1719 Ma (Didenko et al., the handling editor Sergei Pisarevsky, for their constructive cri- 2015) is more consistent with other granitoid-derived poles from tiques of earlier versions. Volodia Pavlov and Roma Veselovskiy southern Siberia (Didenko et al., 2009), but none of those were partly supported by Grants RFBR ##13-05-12030 and 15- granitoid-based data have reliable estimates of paleohorizontal. 35-20599, Ministry of Education and Science of the Russian Feder- Due to this lack of structural control, neither Ulkan result can be ation Grant # 14.Z50.31.0017. Some of the instruments used in the 654 D.A.D. Evans et al. / Precambrian Research 281 (2016) 639–655 present study (JR-6 magnetometers and MFK1-A kappabridge) Proterozoic plate tectonic reconstructions of Siberia and Laurentia. Precambr. were purchased through the Development Program of the Lomono- Res. 89, 1–23. 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